U.S. patent number 4,914,283 [Application Number 07/302,544] was granted by the patent office on 1990-04-03 for circuit arrangement for evaluating the output of a photodiode unit.
This patent grant is currently assigned to Alcatel N.V.. Invention is credited to Willi Brinckmann, Siegfried Nestel.
United States Patent |
4,914,283 |
Brinckmann , et al. |
April 3, 1990 |
Circuit arrangement for evaluating the output of a photodiode
unit
Abstract
The intensity of the optical radiation varies within a wide
range. The proportion of parasitic radiation registered by the
photodiodes (D1 to Dn) increases with increasing radiation
intensity. To exclude this parasitic radiation from evaluation, the
photodiodes (D1 to Dn) are followed by analog comparators (K1 to
Kn) with a variable reference voltage (Uref1). The reference
voltage (Uref1) is varied by an analog comparator (Kz1) in
porportion to the radiation intensity determined by a central
photodiode (Dz).
Inventors: |
Brinckmann; Willi (Tamm,
DE), Nestel; Siegfried (Stuttgart, DE) |
Assignee: |
Alcatel N.V. (Amsterdam,
NL)
|
Family
ID: |
6346138 |
Appl.
No.: |
07/302,544 |
Filed: |
January 26, 1989 |
Foreign Application Priority Data
|
|
|
|
|
Jan 28, 1988 [DE] |
|
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3802450 |
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Current U.S.
Class: |
250/206.1;
250/208.4 |
Current CPC
Class: |
G01S
3/781 (20130101); G01S 3/784 (20130101) |
Current International
Class: |
G01S
3/784 (20060101); G01S 3/78 (20060101); G01S
3/781 (20060101); G01J 001/20 () |
Field of
Search: |
;250/23R,23S,221,208,209,578 ;126/425 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Nelms; David C.
Assistant Examiner: Allen; Stephone B.
Attorney, Agent or Firm: Christie, Parker & Hale
Claims
We claim:
1. A circuit arrangement for determining the angle of incidence of
optical radiation, comprising
a plurality of closely spaced photodiodes which each have a
respective individual field of view corresponding to optical
radiation having a different respective range of said angle of
incidence,
central photodiode means having a combined field of view which
includes all said individual fields of view,
a plurality of analog comparators, each having an input coupled to
a respective one of the outputs from said plurality of photodiodes,
a second input of each of the analog comparators being coupled to a
common reference voltage,
a first analog comparator having an input coupled to the central
photodiode means and a second input also coupled to the common
reference voltage,
reference control means responsive to the output from the central
photodiode means, for varying said common reference voltage in
accordance with the intensity of radiation detected by the central
photodiode.
2. A circuit arrangement as claimed in claim 1, wherein
at least the first analog comparator comprises a respective
operational amplifier functioning as a differential amplifier
having an inverting input and a non-inverting input, with the
common reference voltage being coupled to the inverting input,
and
said reference control means comprises a diode for feeding the
output of that differential amplifier back to the inverting input
thereof and a storage capacitor coupled between said inverting
input and a fixed potential.
3. A circuit arrangement as claimed in claim 1, wherein the output
from the central photodiode means is also coupled to an input of a
second comparator having a second input coupled to a second
reference voltage,
an output of the second comparator is connected to respective
control inputs of a plurality of electronic switches each
respectively associated with a different one of the plurality of
photodiodes, such that when the second comparator responds to a
signal from the central photodiode means indicative of a level of
incident optical radiation above a predetermined upper threshold,
the plurality of electronic switches all assume a first state,
a plurality of load resistors are each coupled to a respective one
of said plurality of electronic switches and to a respective one of
the plurality of photodiodes such that the effective sensitivity of
the respective photodiode is lowered when the respective switch
assumes said first state, and
the output voltage of the first analog comparator is also
responsive to the output of said second comparator, such that when
said effective sensitivity is lowered, a corresponding change is
also made in said first reference voltage.
4. The circuit arrangement of claim 1, wherein
said plurality of closely spaced photodiodes are arranged in a
linear array extending in the direction of a first axis,
said array is disposed behind a surface parallel to said first
axis,
said surface defines a slit extending in the direction of a second
axis, perpendicular to said first axis
said optical radiation passes through said slit before it
illuminates said array, and
said central photodiode means comprises a photodiode located in
front of said surface adjacent to said slit.
Description
TECHNICAL FIELD
The present invention relates to a circuit arrangement having an
array of photodiodes for determining the direction of incidence of
optical radiation.
CLAIM FOR PRIORITY
This application is based on and claims priority from an
application first filed in Fed. Rep. Germany on 01/28/88 under Ser.
No. P38 02 450.0. To the extent such prior application may contain
any additional information that might be of any assistance in the
use and understanding of the invention claimed herein, it is hereby
incorporated by reference.
BACKGROUND ART
A device for determining the direction of incidence of optical
radiation which uses a photodiode unit consisting of a linear array
of photodiodes contained in a light-tight box is disclosed, for
example, in DE-OS published German patent application No. 29 31
818. The wall of the box opposite the the photodiode unit contains
a slit extending transversely to the photodiode array or a suitable
lens by which radiation incident within a predetermined angular
range is focused onto an area of the photodiode unit corresponding
to the angle of incidence. A subsequent circuit evaluates the
signals delivered by the photodiodes. The problems caused by large
differences in radiation intensity, crosstalk between adjacent
photodiodes, and parasitic radiation are not dealt with.
DISCLOSURE OF INVENTION
It is the object of the present invention to provide an arrangement
which ensures precise determination of the direction of incidence
of optical radiation over a wide intensity range.
In accordance with the invention, the photodiodes are followed by
analog comparators coupled to a common reference voltage which is
varied in proportion to the radiation intensity determined by a
central photodiode.
The principal advantages of the invention are that the signals
provided by the photodiodes are evaluated in accordance with the
radiation intensity and, according to a further aspect of the
invention, that when the sensitivity range of the photodiodes is
exceeded, switchover is automatically effected to a range of lower
sensitivity.
BRIEF DESCRIPTION OF DRAWINGS
An embodiment of the invention will now be explained with reference
to the accompanying drawings, in which:
FIG. 1 is a schematic representation of a device for determining
the direction of incidence of optical radiation;
FIG. 2 is a schematic diagram of a circuit arrangement in
accordance with the invention, and
FIG. 3 shows an extended version of the circuit arrangement of FIG.
2 with automatic switchover of the sensitivity range of the
photodiodes.
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a perspective, schematic view of a device for determining
the direction of incidence of optical radiation as is essentially
known from DE-OS 26 03 578. The device is constituted by a
light-tight box 1 whose inside has a low-reflection, e.g. mat
black, surface. (The sidewall facing the viewer has been removed to
show the interior). The front of the box 1 has a vertical slit 2.
Inside the box 1, a photodiode unit DA is mounted transversely to
this slit 2 on the wall opposite the front. It consists of a linear
array of closely spaced photodiodes D1 to Dn.
When optical radiation OR1 or OR2 is incident on the front of the
box 1 as shown schematically in FIG. 1, a narrow portion of the
radiation corresponding to the width of the slit 2, i.e., a portion
OR1' or OR2', respectively, will fall on preferably one of the
photodiodes D1 to Dn.
The photodiodes D1 to Dn are assigned certain angular ranges, so
that when a photodiode responds to incident radiation OR1 or OR2,
the angle of incidence of the radiation and, thus, the direction of
the associated radiation source Q1 or Q2 can be derived. The
maximum angle of view .alpha. is determined essentially by the
depth of the box 1 and the length of the photodiode unit DA.
Depending on the distance between the device of FIG. 1 and the
respective radiation source Q1, Q2, the intensity of the radiation
portion OR1', OR2' incident on the photodiode D1, Dn will be
different. In addition, the intensity distribution over the cross
section is typically a Gaussian distribution. Depending on whether
the slit 2 is struck by the radiation OR1, OR2 completely, as shown
schematically in FIG. 1, or only in part, the intensities of the
radiation portions incident on the photodiodes D1 to Dn will differ
widely even if the radiation sources are located at the same
distance from the device. Furthermore, when passing through the
slit 2, the radiation undergoes diffraction, which increases with
increasing radiation intensity. The radiation portions OR1', OR2'
incident on the photodiodes D1, Dn may thus be considerably wider
than a photodiode. In conjunction with additional electrical and/or
optical crosstalk and/or due to reflection within the box, this
effect leads to output signals which permit unambiguous evaluation
only within a limited intensity range of the radiation unless
special steps are taken.
Accordingly, a circuit had to be provided which, taking account of
the dynamics of the radiation acting on the photodiodes D1 to Dn
and of spurious side effects, ensures unambiguous determination of
the direction of incidence of radiation and has an evaluation range
several times as wide as in conventional designs.
Two such circuits are shown in FIGS. 2 and 3. Each of them consists
of a signal portion SB and a central portion ZB. The signal portion
SB contains the photodiode unit DA with the photodiodes D1 to Dn
and signal channels following the photodiodes. The central portion
ZB contains a central photodiode Dz followed by a central channel.
The central diode Dz is disposed so that its field of view is equal
to the field of view of the photodiode unit DA (angle of view
.alpha. in FIG. 1). The inputs of the photodiodes D1 to Dn and Dz
are connected to a bias voltage Uv.
As shown in FIG. 2, each signal channel contains a load resistor
R1.multidot.1, . . . , Rn.multidot.1, a high-pass filter
C1/R1.multidot.3, . . . , Cn/Rn.multidot.3, an operational
amplifier V1, . . . , Vn, and an analog comparator K1, . . . , Kn.
The analog comparators are preferably operational amplifiers used
as differential amplifiers whose inverting inputs are connected to
a reference voltage Uref1. The outputs of the comparators K1 to Kn
are connected to a processor-controlled signal evaluation unit (not
shown). The central channel contains a load resistor Rz 1, a
high-pass filter Cz/Rz 2, and an analog comparator Kz1, preferably
an operational amplifier used as a differential amplifier, whose
output is connected to positive potential U+ through a load
resistor Rz3. The output of the comparator Kz1 is fed through a
diode Ds back to the inverting input, which is connected to the
reference voltage Uref1. A storage capacitor Cs has one terminal
connected to the reference-voltage lead running to the comparator
Kz1, and the other grounded. The values of the load resistors
R1.multidot.1 to Rn.multidot.1 are chosen so that the basic
sensitivity of the photodiodes D1 to Dn is high, so that they
already respond to optical radiation of low intensity. The
following highpass filters C1/R1.multidot.3 to Cn/Rn.multidot.3 are
designed to filter out continuous radiation or pulses of long
duration, so that only the short-duration radiation pulses of
interest will be evaluated. The amplifiers V1 to Vn are designed to
raise the signal level linearly to a level related to the reference
voltage Uref1. While the high-pass filter Cz/Rz 2 in the central
channel is identical to the high-pass filters in the signal
channels, the value of the load resistor Rz 1 is chosen so that the
basic sensitivity of the central diode Dz is lower than that of the
photodiodes D1 to Dn.
As described above, the central diode Dz covers a field equal to
the total field of view of the photodiodes D1 to Dn. The central
diode Dz is therefore located close to the slit 2 as shown in FIG.
1, so that a radiation pulse a portion of which falls on any of the
photodiodes D1 to Dn will strike the central photodiode Dz
completely. Because of the aforementioned diffraction of the
radiation portion passing through the slit 2 (FIG. 1), the
electrical and/or optical crosstalk, and the reflection within the
box, not only the directly illuminated photodiode D1, Dn but, as a
rule, also adjacent photodiodes will respond, depending on the
intensity of the radiation pulse. This parasitic radiation, in
response to which the signal channels following those adjacent
photodiodes deliver voltage pulses of different magnitude, is
evaluated by the analog comparators K1 to Kn. The latter, however,
respond only to voltage values above the applied reference voltage
Uref1. This reference voltage Uref1 is chosen so that a maximum of
two, generally adjacent, comparators K1 to Kn will respond, whose
output signals are weighted by the subsequent evaluation unit and
from which the exact angle of incidence of the radiation pulse is
determined, e.g., by interpolation.
The intensity of the parasitic radiation increases with increasing
intensity of the radiation pulses. Before the parasitic radiation
reaches the order of magnitude of the reference voltage Uref1, the
central photodiode Dz, in response to the incident radiation,
delivers a voltage greater than the reference voltage Uref1. As a
result, the analog comparator Kz1 responds as long as the increased
voltage is applied, its output voltage varying in proportion to the
difference between the output potential of the central photodiode
Dz and the reference voltage. The output voltage, which thus varies
proportionally with the intensity of the detected radiation pulse,
is fed back to the inverting input of the comparator Kz1 through
the diode Ds. By being superimposed on the fixed reference voltage
Uref1, it becomes the new reference voltage for the analog
comparators K1 to Kn. Because of the short radiation pulses, the
storage capacitor Cs is charged to temporarily maintain the
increased reference voltage. By the new reference voltage, which
varies proportionally with the radiation intensity, the thresholds
of the comparators K1 to Kn are raised to such a level that signals
caused by parasitic radiation continue to be suppressed.
The radiation pulses to be detected may exhibit dynamics which go
far beyond the sensitivity range of the photodiodes D1 to Dn, which
is determined by the preset high basic sensitivity and the upper
sensitivity limit. If no special steps are taken, only a part of
the range of variation of the radiation pulses can be covered with
a photodiode unit DA, so that radiation pulses of low intensity,
for example, will not be detected because the comparators K1 to Kn
will not respond, In the presence of high-intensity radiation
pulses, signal voltages caused by parasitic radiation may become
higher than the threshold voltages of the associated comparators K1
to Kn, so that the latter will respond. In that case, precise
determination of the angle of incidence of the associated radiation
pulses is no longer ensured.
FIG. 3 shows an extended version of the circuit arrangement of FIG.
2 in which the sensitivity range of the photodiodes D1 to Dn is
shifted within the range of variation of the incident radiation by
switching to a lower basic sensitivity so that radiation pulses of
higher intensity will also be reliably evaluated. To this end, a
resistor R1.multidot.2, Rn.multidot.2 can be connected in parallel
with each of the load resistors R1.multidot.1 to Rn.multidot.1 of
the photodiodes D1 to Dn via an electronic switch S1, Sn. The
central portion ZB includes a second comparator Kz2 which, like the
first comparator Kz1, has its noninverting input connected to the
output of the central photodiode Dz. The inverting input of the
comparator Kz2 is connected to a reference voltage Uref2 which
causes the comparator Kz2 to respond to a signal from the central
photodiode Dz only if and as long as the intensity of the detected
radiation pulse reaches or exceeds the upper sensitivity limit of
the photodiodes D1 to Dn. Connected to the output of the first
comparator Kz1 is an electronic switch Sz in series with a grounded
resistor Rz4. In the ON state of the switch Sz, the two resistors
Rz3 and Rz4 form a voltage divider via which the operating point of
the first comparator Kz1 is changed.
As the second comparator Kz2 responds, the switches S1 to Sn and Sz
turn on. In this state, the load resistances of the photodiodes D1
to Dn consist of the parallel combination of the resistors
R1.multidot.1 and R1.multidot.2 to Rn.multidot.1 and Rn.multidot.2.
The resulting resistances are equal and such that the sensitivity
range of the photodiodes D1 to Dn is shifted within the dynamic
range of the radiation to be detected, as described above. At the
same time, the output voltage of the first comparator Kz1, which is
also the reference voltage for the analog comparators K1 to Kn, is
reduced via the voltage divider Rz3/Rz4 in such a way that the
thresholds of the comparators K1 to Kn are lowered in proportion to
the decrease in the sensitivity of the photodiodes D1 to Dn.
* * * * *